Persistence in a fragmented habitat: genetic diversity and pollination patterns in Asclepias viridiflora

Habitat loss and fragmentation are expected to erode genetic variation and contribute to genetic differentiation by limiting gene flow among isolated habitat patches. Yet, isolated plant populations often retain genetic diversity and exhibit limited population genetic structure. Using an assessment of genetic diversity and pollination patterns in Asclepias viridiflora across 6400 ha of fragmented prairie remnants in western Minnesota (USA), we aimed to (1) characterize the spatial genetic structure of A. viridiflora in a fragmented landscape and (2) evaluate if pollen movement contributes to gene flow among isolated patches. This spatial scale of our sampling area allowed us to detect pollination events over several kilometres, if they occurred.

We mapped, sampled and genotyped 102 sexually mature plants and 179 of their progeny using 19 microsatellite loci, including several new loci discovered using genomic tools. The unusual pollination system of milkweeds aided paternity assignment since we could genotype multiple offspring with shared paternity. We assessed genetic diversity and genetic structure and used paternity analysis to characterize spatial mating patterns using high-resolution mapping.

Asclepias viridiflora in this fragmented landscape retained high levels of genetic diversity (HE, expected heterozygosity, averaged 0.69 across loci) and minimal population genetic structure, indicating genetic degradation has not occurred. Paternity assignment revealed that most (74.4 %) pollination events involved pollen transfer over relatively short distances (i.e. within 70 m). However, surprisingly long-distance pollinations (up to 9 km) also occurred, and there was evidence of pollen flow from outside our study area.

We found little evidence for population structure or reduced genetic variability across scattered clusters of A. viridiflora within our study area. Occasional long-distance pollination events, which we identified, may be sufficient to maintain reproductive connectivity over substantial areas despite severe habitat fragmentation. Our findings suggest that even small, spatially isolated clusters of plants may not be reproductively isolated and may contribute to the persistence of fragmented plant populations.